Landscape irrigation with salty water

S. Miyamoto and Rick Galceran and Richard Garcia

Managers and superintendents of golf courses, parks, school grounds and
other large landscape installations face increasing public and budgetary
demands to use reclaimed sewage water or salty non-potable water for
irrigation. Such pressures are especially strong and politically popular in
water-short areas of the arid Southwest. However, some grounds managers
make the decision to switch from potable to salty non-potable water for
irrigation without an adequate understanding of the various constraints or
the potential for soil degradation.

Most grounds-care professionals are aware of the potential for problems
stemming from the use of salty water. However, many factors other than
water quality affect the degree of hazard, making such appraisal difficult.
Here, we'll outline some key factors that affect appraisal of potential
salt problems from using salty water for irrigating landscapes in the arid
Southwest. Much of the information we present here is the result of a
project supported by the U.S. Bureau of Reclamation and a cooperative
agreement between El Paso Water Utilities and the Texas A&M University
Agricultural Research Center at El Paso.

Effects of excess salt
Salt affects growth, vigor and appearance of landscape plants in at least
two ways. The first is damage caused by foliar contact with salts, and the
second is the damage induced by exposure of roots to salty soil solutions.

Foliar-induced salt damage to leaves is common in shrubs, trees and flowers
irrigated with sprinklers and can occur on broadleaf plants at
dissolved-salt contents (principally sodium and chlorine) as low as 500
ppm, especially with frequent use of mist-forming sprinklers.

Concentrations above 1,000 ppm severely affect many shrubs and trees when
directly sprinkled on their foliage. Turf species, however, are relatively
tolerant of this form of salt damage (excepting Dichondra and possibly some
salt-sensitive bluegrass species). The extent of salt-induced foliar damage
varies widely according to plant species, frequency and timing of
irrigation, sprinkler type and weather conditions. Fortunately, managers
can readily correct such problems with modification of their irrigation
schedules or systems.

Root-induced salt damage also can occur in most plants. With some
exceptions, plant material prevalent in the Southwest tolerates greater
salinity than the existing levels in reclaimed sewage water or salty
non-potable water (see Table 1, opposite page). Therefore, root-induced
salt problems should, in theory, only occur in situations where dissolved
salt from irrigation water accumulates or in soil that is inherently
saline. In the Southwest, the first scenario prevails, as irrigation water
in this region contains appreciable amounts of salts, typically 500 to
1,500 ppm. Irrigation with such water sources brings in 3 to 12 tons of
salt per acre per year at a typical annual irrigation rate of 4 feet.

The salts deposited through irrigation must be leached for sustained growth
of landscape plants. Otherwise, soil salinity rapidly will increase with
repeated irrigations. Table 2 (below right) shows the drainage (leaching
fraction) necessary to keep soil salinity at or below the target level. The
term leaching fraction (LF) refers to the fraction of water that drains
relative to the amount you apply. Higher salinity in irrigation water
demands a greater LF, as do plants with low salinity tolerance. In essence,
an appraisal of potential salt problems is a consideration of whether you
can maintain an adequate LF on a sustainable basis.

Salts may affect plant growth in other, indirect ways. These include sodium
effects, which reduce water penetration and also can increase waterlogging
or water stress. Other indirect effects include difficulties with nutrient
uptake and potential increases in salt-tolerant weeds. However, these
effects are beyond the scope of this article.

Experiences of golf courses
Many golf courses in the Southwest and Southern California already use
reclaimed sewage water or salty non-potable water for irrigation. El Paso
Water Utilities and the consulting firm of CH2M Hill (Albuquerque, N.M.)
recently surveyed 13 golf courses using water sources with differing levels
of salinity (see Table 4, page 24, for summary of results).

The two golf courses using reclaimed municipal sewage water with dissolved
salts of less than about 500 ppm reported no problems.

Three out of seven courses in the survey that used water with 500 to 1,000
ppm dissolved salts reported some, but not widespread, salt problems. These
include Coronado (El Paso, Texas) and two courses in Southern California
(Tustin Ranch and Harding Park). Salt problems at the latter two courses
are confined at the moment to greens with salt-sensitive bentgrass
varieties. At Coronado, sporadic salt problems are occurring on fairways
and roughs on soils containing an undisturbed layer of hardened caliche.
Soil salinity in such areas increased to 6 to 13 dS m-1 in the saturation
extract, which is high enough to cause salt damage in many species. Four
other Southern California golf courses included in this survey (but not
listed in Table 4) reported no salt problem. However, these golf courses
use low-salt water for greens, and some of them do not have a long enough
history to assess the real salt hazard.

The three golf courses we surveyed that use water with more than 1,000 ppm
each reported salt problems.

1. At a Los Angeles course, these were limited to bentgrass greens and to
fairways on low-permeability clay soils.

2. Salt problems at a course near El Paso were mostly confined to some high
ground where construction activity removed the topsoil over a caliche layer
during construction. In addition, many trees that received direct
sprinkling with salty water suffered extensive salt-induced foliar damage
(see photo, page 24). However, a high level of salt accumulation also
occurred in one area where coarse sand underlaid 7 inches of loamy soil
(see photo, opposite page). This type of artificial, layered soil profile
makes water penetration and salt leaching difficult unless you adjust
irrigation depths.

3. Salt problems at another West Texas course have been widespread, and the
course has required major renovation. Salts accumulated at the ground
surface, including greens, even though the greens are sand-based and the
fairways are on deep loamy sand. Trees and shrubs at this course also
sustained salt-induced foliar damage.

Soil-salinity surveys
Appraisal of the potential for salt problems based on water quality is
useful, especially for appraising salt-induced foliar damage. However, it
leaves many unanswered questions about long-term soil-salinization
potential, which is influenced by soil quality, especially soil
permeability.

Parks and schools. With this in mind, we surveyed the soil-salinity status
of city parks and schools on seven different soil series in the El Paso
area. We had previously sampled most of these same sites in 1978, providing
a long-term basis for comparison (see Table 3, at left). We took soil
samples in the spring of 1997 from what we considered to be a
representative area of each site and analyzed them for salinity of the
saturation extract. All the grounds we surveyed had common bermudagrass and
have been irrigated for 20 years or more using water with 600 to 800 ppm of
dissolved salts and a sodium adsorption ratio (SAR) of 5.5 to 6.5.

The survey showed that high levels of salt accumulation occurred in the
Harkey and Glendale series. These soils have texture ranging from silt loam
to silty clay loam. The permeability of silty clay loam or silty clay is
inherently low and seems to have gotten worse at these sites due to soil
compaction and the precipitation of calcium salts in soil pores. The
condition of the turf in these areas is not good, and salt leaching seems
to have ceased for all practical purposes.

This situation is similar to using salty water to refill a pan as its water
evaporates: Water leaves the pan via evaporation, but the dissolved salt
does not. Each time you refill, you add more salt and the water becomes
saltier. Thus, the use of water with higher salinity is not advisable at
sites such as these unless you can find an economical way of improving soil
permeability.

Additionally, we found some silt-loam areas that had a water table within
30 inches of the surface, even during winter when no irrigation was
occurring. This situation prevents leaching and therefore precludes the use
of salty irrigation water without a practical way to cope with the high
water table.

We also saw high levels of salt accumulation in limited areas of the
Delnorte and Hueco series where a hardened caliche layer remained
undisturbed (similar to some areas of the Coronado course located on
Delnorte soil). The salt readings there already are high enough to affect
growth of intolerant plant species and will rise further with continued use
of higher-salinity water for irrigation.

Soil salinity generally increases in proportion to irrigation-water
salinity. However, we found no significant salt accumulation in Bluepoint
and Canutio soil series, which are deep, well-drained sandy or gravelly
soils. These findings indicate that soil texture and the presence of a
caliche layer play a major role in salt accumulation by inhibiting leaching.

Golf courses. In addition, we surveyed the soil-salinity status at two golf
courses: one that has been irrigated for 35 years with salty water (1,200
to 1,600 ppm) and another along transects with slopes ranging from 15 to 30
percent.

Soil-salinity readings from the former were quite variable, but the
variability was higher on higher ground. This may have been due to the
effects of grading as well as non-uniform irrigation patterns. Also, the
surface of this soil type has small mounds of wind-blown loamy sand and
sandy loam that might have filled low spots during grading. The coefficient
of variability in salinity readings at high and low grounds was 35 and 14
percent respectively, while the mean salinity did not differ a great deal.
This means that complex topography increases variability and can lead to
development of salt spots, as we also observed at the other course we
surveyed. If you want to determine a meaningful average salinity of low and
high grounds, you'll need to take three to ten samples from each.

Suggestions for your situation
Salts can adversely affect growth, vigor and appearance of landscape plants
through foliar contact via irrigation water, root exposure to salty soil
solutions and through indirect effects. A survey of golf courses in the
Southwest and Southern California indicates sporadic salt problems on
greens and fairways, with poor drainage or poor water infiltration when
irrigation-water salinity exceeds about 500 ppm. Above 1,000 ppm, salt
problems are common and include both foliar- and root-induced damages.
Foliar-induced damage can be quite extensive when water with salinity
exceeding 1,000 ppm is sprinkled directly on trees and shrubs.

Long-term salt accumulation is highly soil-dependent, and in soils with
texture finer than loam and soils with a caliche layer it can reach 20 to
40 dS m-1 in a matter of 20 years, even when you use relatively low-salt
water (600 to 800 ppm) for irrigation. This means that assessing potential
salt problems based on water-quality testing alone is too simplistic. You
should check soil salinity and leachability of salts before using salty,
non-potable water for irrigation, using a soil map and topographical
features as a framework for sampling. Remember that complex topography and
steep slopes tend to increase soil-salinity variability. Thus, you'll need
3 to 10 soil samples per area consisting of the same soil type and
topography for a credible assessment of soil salinity. In soils with poor
salt leaching, the grounds manager must weigh the benefit of using salty
non-potable water against the cost of amending the soil to make it more
permeable, drainable and manageable. Once you make the decision to use
salty water, you should conduct periodic soil-salinity monitoring to
evaluate salinization trends and make management adjustments.

Dr. S. Miyamoto is professor of soil and water science at the Texas A&M
University Agricultural Research Center (El Paso, Texas); Ricardo Galceran
is manager of biosolids and water re-use for El Paso Water Utilities; and
Richard Garcia is manager of parks operations, Parks and Recreation
Department, City of El Paso.